The first place I check when a double has stiff opening is the chambers. If the chambers are dirty they put a lot of drag on the extracting shells and make you think there is a mechanical problem with the gun. I have corrected stiff opening guns several times now by putting a piece of green Scotchbrite on the end of a rod and using a hand drill to scour the chambers clean. Use good lube with this to float the grit out.
I am not real familiar with your particular gun but there are a number of less expensive doubles I have encountered that need to wear themselves in a bit. Make sure the mating surfaces are well lubed. Wipe that lube out after each use and relube because if it is wearing itself in there has to be some metal cutting going on and you want to get the debris off those surfaces.
